1. Field of the Invention
The invention relates to an improved scroll-type compressor and more particularly, to an improved scroll-type compressor with a variable displacement mechanism.
2. Description of the Prior Art
A scroll-type compressor which can vary the compression ratio is well known in the art. A scroll-type compressor with a variable displacement mechanism is depicted in FIGS. 1(a) and 1(b). This compressor's variable displacement mechanism is similar to the variable displacement mechanism described in Japanese Utility Model Application Publication No. 63-177688.
As depicted in FIGS. 1(a) and 1b), a bypass passage 40 includes a bypass hole 41 formed in a first plate 11 of a fixed scroll member 10, and a side bypass passage 42 which also is formed in first plate 11 and extends in a radial direction to first plate 11. A cylinder 50 is coaxial with side bypass passage 42, and, therefore, a shuttle valve member 60, which is slidably disposed in cylinder 50 and side bypass passage 42, is also coaxial with side bypass passage 42. In addition, a spring 70 biasing shuttle valve member 60 is disposed in side bypass passage 42.
The pressure in cylinder 50 is controlled by adjusting pressure applied against an end 60a of shuttle valve member 60. The position of shuttle valve member 60 is controlled for opening and closing bypass passage 40 by utilizing the relationship between the adjusted pressure applied against end 60a and the force of spring 70 biasing shuttle valve member 60.
For this purpose, the compressor in FIGS. 1(a) and 1(b) is provided with a discharge pressure (Pd) passage 103 for introducing fluid from a discharge chamber (not shown) into cylinder 50, and is also provided with a suction pressure (Ps) passage 104 for returning the fluid in cylinder 50 to a suction chamber 29. An orifice 105 is provided in Pd passage 103, so that a reduced Pd is always introduced into cylinder 50. Meanwhile, a device for controlling the pressure (not shown) between Ps passage 104 and Pd passage 103 is provided in Ps passage 104. This device selectively opens and closes Ps passage 104 to adjust the displacement of the compressor.
Therefore, the force applied to opposite ends 60a and 60b of shuttle valve member 60 has the relationship set forth below. When Ps passage 104 is opened, the end of cylinder 50 nearest Ps passage 104 is placed in communication with suction chamber 29, the fluid in cylinder 50 immediately flows through Ps passage 104 into suction chamber 29. The displacement of the compressor, thus, changes from the maximum to the minimum value. Assuming that:
Pc is the control pressure introduced into cylinder 50, PA1 Pm is the pressure of the fluid being compressed in a fluid pocket (not shown), PA1 Pd is the discharge pressure, PA1 Ps is the suction pressure, and PA1 F is the spring force of spring 70; PA1 p is the difference between the forces applied to opposite ends 60a and 60b of shuttle valve member 60 and is expressed as follows: EQU P=Pc-Ps+F.
Consequently, when Pc=Ps, only spring force F acts to open shuttle valve member 60. This results in a problem relating to the responsiveness of shuttle valve member 60 in cylinder 50.
In this configuration, when the movement of shuttle valve member 60 opens bypass passage 40, the fluid which is compressed in the fluid pocket immediately returns through bypass passage 40 to suction chamber 29. Therefore, when shuttle valve member 60 opens bypass passage 40, the fluid, passes over end 60b of shuttle valve member 60 and immediately flows through bypass passage 40 into suction chamber 29. Thus, end 60b of shuttle valve member 60 receives little pressure from the compressed fluid. Further, because spring 70 for biasing shuttle valve member 60 open is disposed in bypass passage 40, spring 70 causes a pressure loss when the fluid flows through bypass passage 40 into suction chamber 29.